Planar Proton Minibeam Irradiation Elicits Spatially Confined DNA Damage in a Human Epidermis Model

Proton-FLASH: effects of ultra-high dose rate irradiation on an in-vivo mouse ear model

FLASH-radiotherapy may provide significant sparing of healthy tissue through ultra-high dose rates in protons, electrons, and x-rays while maintaining the tumor control. Key factors for the FLASH effect might be oxygen depletion, the immune system, and the irradiated blood volume, but none could be fully confirmed yet. Therefore, further investigations are necessary. We investigated the protective (tissue sparing) effect of FLASH in proton treatment using an in-vivo mouse ear model. The right ears of Balb/c mice were irradiated with 20 MeV protons at the ion microprobe SNAKE in Garching near Munich by using three dose rates (Conv = 0.06 Gy/s, Flash9 = 9.3 Gy/s and Flash930 = 930 Gy/s) at a total dose of 23 Gy or 33 Gy. The ear thickness, desquamation, and erythema combined in an inflammation score were measured for 180 days. The cytokines TGF-β1, TNF-α, IL1α, and IL1β were analyzed in the blood sampled in the first 4 weeks and at termination day. No differences in inflammation reactions were visible in the 23 Gy group for the different dose rates. In the 33 Gy group, the ear swelling and the inflammation score for Flash9 was reduced by (57 ± 12) % and (67 ± 17) % and for Flash930 by (40 ± 13) % and (50 ± 17) % compared to the Conv dose rate. No changes in the cytokines in the blood could be measured. However, an estimation of the irradiated blood volume demonstrates, that 100-times more blood is irradiated when using Conv compared to using Flash9 or Flash930. This indicates that blood might play a role in the underlying mechanisms in the protective effect of FLASH.

Postirradiation temperature influences DSB repair and dicentric chromosome formation-potential impact for dicentric chromosome analysis in interlaboratory comparisons

The objective was to investigate the influence of different pre-storage temperatures in the dicentric chromosome analysis (DCA) protocol (22°C vs. 37°C) by using γ-H2AX + 53BP1 foci as a marker for deoxyribonucleic acid (DNA) double-strand break (DSB) damage induction and repair and the formation of dicentric chromosomes as a result of mis-repair. Repair of γ-H2AX + 53BP1 DSB foci was absent in samples that were incubated for 2 h at 22°C after exposure of 0.5 and 1.2 Gy. When 0.5- and 1.2-Gy-exposed samples were incubated at 37°C for 2 h, there was an average decline of 31 and 52% of DSB foci, respectively. This indicated that DNA repair occurred. There was a 27% decrease in dicentric chromosome yield at 1.2 Gy and a 15% decrease at 3.5 Gy after post-irradiation incubation for 2 h at 37°C relative to the observed dicentric frequencies at 22°C. Recommended to re-phase: our data suggested that there were more open DSBs after a 2-h incubation at 22°C, which contributed to more mis-repair and dicentric formation from the start of culture. Our findings are corroborated by publications showing that lesion interaction based on enzymatic activity is suppressed below 21°C. As such temperature variations can be a source of variation in DCA during interlaboratory comparison studies, we propose to establish a common guide for the standardisation of pre-culture conditions in cytogenetic dosimetry proficiency testing.

Analysis of Single- and Double-Stranded DNA Damage in Osteoblastic Cells after Hyperbaric Oxygen Exposure

(1) Background: Hyperbaric oxygen (HBO) exposure induces oxidative stress that may lead to DNA damage, which has been observed in human peripheral blood lymphocytes or non-human cells. Here, we investigated the impact of hyperbaric conditions on two human osteoblastic cell lines: primary human osteoblasts, HOBs, and the osteogenic tumor cell line SAOS-2. (2) Methods: Cells were exposed to HBO in an experimental hyperbaric chamber (4 ATA, 100% oxygen, 37 °C, and 4 h) or sham-exposed (1 ATA, air, 37 °C, and 4 h). DNA damage was examined before, directly after, and 24 h after exposure with an alkaline comet assay and detection of γH2AX+53BP1 colocalizing double-strand break (DSB) foci and apoptosis. The gene expression of TGFß-1, HO-1, and NQO1, involved in antioxidative functions, was measured with qRT-PCR. (3) Results: The alkaline comet assay showed significantly elevated levels of DNA damage in both cell lines after 4 h of HBO, while the DSB foci were similar to sham. γH2AX analysis indicated a slight increase in apoptosis in both cell lines. The increased expression of HO-1 in HOB and SAOS-2 directly after exposure suggested the induction of an antioxidative response in these cells. Additionally, the expression of TGF-ß1 was negatively affected in HOB cells 4 h after exposure. (4) Conclusions: in summary, this study indicates that osteoblastic cells are sensitive to the DNA-damaging effects of hyperbaric hyperoxia, with the HBO-induced DNA damage consisting largely of single-strand DNA breaks that are rapidly repaired.

Initial experience on abdominal photon-counting computed tomography in clinical routine: general image quality and dose exposure

OBJECTIVES

Photon-counting computed tomography has lately found its way into clinical routine. The new technique could offer substantial improvements regarding general image quality, image noise, and radiation dose reduction. This study evaluated the first abdominal examinations in clinical routine and compared the results to conventional computed tomography.

METHODS

In this single-center retrospective study, 66 patients underwent photon-counting and conventional abdominal CT. Four radiologists assessed general image quality, image noise, and image artifacts. Signal-to-noise ratio and dose properties of both techniques within the clinical application were compared. An ex vivo phantom study revealed the radiobiological impact by means of DNA double-strand break foci in peripheral blood cells by enumerating γ-H2AX+53BP1 foci.

RESULTS

General image quality in accordance with the Likert scale was found superior for photon-counting CT (4.74 ± 0.46 vs. 4.25 ± 0.54; p < 0.001). Signal-to-noise ratio (p < 0.001) and also dose exposure were higher for photon-counting CT (DLP: 419.2 ± 162.2 vs. 372.3 ± 236.6 mGy*cm; p = 0.0435). CT exposure resulted in significantly increased DNA damage in comparison to sham control (p < 0.001). Investigation of the average foci per cell and radiation-induced foci numbers revealed significantly elevated numbers (p = 0.004 and p < 0.0001, respectively) after photon-counting CT.

CONCLUSION

Photon-counting CT in abdominal examinations showed superior results regarding general image quality and signal-to-noise ratio in clinical routine. However, this seems to be traded for a significantly higher dose exposure and corresponding double-strand break frequency. Optimization of standard protocols in further clinical applications is required to find a compromise regarding picture quality and dose exposure.

KEY POINTS

• Photon-counting computed tomography promises to enhance the diagnostic potential of medical imaging in clinical routine. • Retrospective single-center study showed superior general image quality accompanied by higher dose exposure in initial abdominal PCCT protocols compared to state-of-the-art conventional CT. • A simultaneous ex vivo phantom study revealed correspondingly increased frequencies of DNA double-strand breaks after PCCT.

Repair of α-particle-induced DNA damage in peripheral blood mononuclear cells after internal ex vivo irradiation with 223Ra

Purpose

As α-emitters for radiopharmaceutical therapies are administered systemically by intravenous injection, blood will be irradiated by α-particles that induce clustered DNA double-strand breaks (DSBs). Here, we investigated the induction and repair of DSB damage in peripheral blood mononuclear cells (PBMCs) as a function of the absorbed dose to the blood following internal ex vivo irradiation with [²²³Ra]RaCl₂.

Methods

Blood samples of ten volunteers were irradiated by adding [²²³Ra]RaCl₂ solution with different activity concentrations resulting in absorbed doses to the blood of 3 mGy, 25 mGy, 50 mGy and 100 mGy. PBMCs were isolated, divided in three parts and either fixed directly (d-samples) or after 4 h or 24 h culture. After immunostaining, the induced γ-H2AX α-tracks were counted. The time-dependent decrease in α-track frequency was described with a model assuming a repair rate R and a fraction of non-repairable damage Q.

Results

For 25 mGy, 50 mGy and 100 mGy, the numbers of α-tracks were significantly increased compared to baseline at all time points. Compared to the corresponding d-samples, the α-track frequency decreased significantly after 4 h and after 24 h. The repair rates R were (0.24 ± 0.05) h⁻¹ for 25 mGy, (0.16 ± 0.04) h⁻¹ for 50 mGy and (0.13 ± 0.02) h⁻¹ for 100 mGy, suggesting faster repair at lower absorbed doses, while Q-values were similar.

Conclusion

The results obtained suggest that induction and repair of the DSB damage depend on the absorbed dose to the blood. Repair rates were similar to what has been observed for irradiation with low linear energy transfer.